1. Field of the Invention
The invention relates to the field of transducers of the strain gage type.
2. Prior Art
It is well-known to fabricate pressure transducers by defining patterns or regions of materials having strain gage characteristics on a flexible diaphragm. For the most part, commercial embodiments of these transducers are either diffused regions in a monocrystalline diaphragm or thin films on a metal or other diaphragm.
The semiconductor approach to these transducers (diffused regions in a monocrystalline substrate/diaphragm) has the advantage of using well-developed semiconductor processing. This processing includes such steps as growing oxide layers, chemically depositing layers, masking, and chemical and plasma etching. These steps allow the simultaneous fabrication of many identical transducers and compensating circuits, including active networks on the same substrate at a relatively low cost. For an example of such fabrication, see U.S. Pat. Nos. 3,764,950 and 4,033,787. These devices, however, have some drawbacks. The current density in the doped regions can be sufficiently high to cause continued diffusion of the dopant into the substrate which changes the properties of the gage. The deflection of the substrate causes relocation of crystal faults which influences carrier mobility. Ions trapped in junctions cause conductivity changes which also affect stability.
Thin film metal strain gage materials do not exhibit instabilities due to these effects. However, the fabrication process, such as electroplating and electroetching, are more expensive than the semiconductor processing techniques and do not lend themselves to the high volume processing techniques used in semiconductor wafer processing.
In some cases, a concentric boss is included on the diaphragm so as to confine most of the deflection to the outer portion of the diaphragm. This improves the linearity and sensitivity of the transducer. (See U.S. Pat. No. 3,341,794.) This same result is obtained by forming the active strain gage resistors over thinner sections of a diaphragm (beam) such as shown in U.S. Pat. No. 3,520,191.
As will be seen, the transducer of the present invention has the performance advantages of the thin film-type transducers, and the processing advantages of the semiconductor-type transducers. The present invention also provides a process which allows platinum to be etched in the presence of other metals.